The present invention relates to a method for galvanically producing pressing matrices which are to be used for producing foils, particularly grinding foils which are capable of rotating with very little eccentricity between the groove turns and the axis of rotation on a playback instrument for picture records and which serve to grind a worn pickup to restore its desired shape.
Pickups for picture records have a shorter lifetime than pickups for phonograph records as a result of the relatively high information density associated with the former. It has therefore been found advisable to restore the pickup to its original, desired shape, after a certain period of use, by a grinding process. This can be effected either automatically by a grinding device built directly into the playback device, or the grinding may be performed at a central location. In either case it is necessary to have available grinding belts or foils of suitable quality.
It has been discovered that the grinding foils produced according to prior art methods do not rotate sufficiently concentrically, i.e. wih sufficiently small eccentricity between the groove turns and the axis of rotation, to produce high quality grinding at the required high rate of revolution, which is of the order of 10,000 rpm.
It is known to produce centered matrices for phonograph and video records from a lacquer foil in that a centering core having a galvanically reproduceable centering edge is inserted into the lacquer foil whereupon the groove is cut and then a matrix is produced therefrom by galvanic means.
The matrix with the reproduced profile of the centering core is then built into the pressing mold.
This process, however, requires cutting the center hole out of the foil upon completion of the stamping process, thus producing a further possibility for error and the creation of eccentricities. The high rates of revolution of about 10,000 rpm required for the grinding of pickups, require greater precision. This precision cannot be attained with the prior art methods.